Digital cameras, scanners, and other imaging devices often use image sensors, such as charge-coupled device (CCD) image sensors or complementary metal-oxide-semiconductor (CMOS) image sensors, to convert optical signals to electrical signals. An image sensor typically includes a grid of pixels, row access circuitry, column access circuitry, a ramp signal generator, and comparators. The pixels capture the light impinged on them and convert the light signals to electrical signals. The converted signals are usually analog signals. The row access circuitry controls which row of pixels that the sensor will read. The column access circuitry includes column read circuits that read the signals from corresponding columns. The ramp signal generator generates a ramping signal as a global reference signal for column read circuits to record the converted electrical signal. Comparators compare the ramping signals and the readout signals from the pixels, and convert the readout signals from analog signals to digital signals for representing the sensed pixels in a digital format.
In a typical image sensor circuit, a large number of comparators (e.g., thousands) may be used such that the conversion of readout signals can be performed in parallel. Because a conventional comparator receives analog signals as input signals, it naturally uses analog circuits as its basic building components. Analog circuits typically include analog transistors having minimum feature sizes that are greater than those of digital transistors.
Moreover, because a large number of comparators are used, a significant portion of the total power consumption and the vertical fixed-pattern noise (VFPN) of an image sensor circuit may be attributed to the comparators. Additionally, analog type comparators may have a slower conversion speed, due to the low operation speed of analog transistors.